In recent years, there is a growing demand for greater-volume and faster radio communication, and studies are being actively carried out on methods of improving an effective utilization rate of limited frequency resources. As one such method, techniques using a space region are attracting attention.
A MIMO (Multiple Input Multiple Output) technology provides a plurality of antenna elements for both a transmitter and receiver and realizes spatially multiplexed transmission in a propagation environment with low correlativity of received signals between antennas (see Non-Patent Literature 1). In this case, the transmitter transmits a data sequence which differs from one antenna element to another from the plurality of attached antennas at the same time, the same frequency and using physical channels of the same code. The receiver separates and receives different data sequences from a received signal from the plurality of attached antennas based on a propagation channel estimation result. Using a plurality of such spatially multiplexed channels, it is possible to achieve faster data transmission without using M-ary modulation. In an environment when there are many scatterers between the transmitter and receiver under a condition with a sufficient S/N (signal-to-noise power ratio), if the same number of antennas are provided between the transmitter and receiver, it is possible to expand the communication capacity in proportion to the number of antennas.
Furthermore, as a different MIMO technology, a multiuser MIMO technology (Multiuser-MIMO or MU-MIMO) is known. The MU-MIMO technology is already being discussed in relation to a next-generation radio communication system standardization standard. The 3GPP-LTE standard or IEEE802.16m standard draft, for example, incorporates a transmission scheme based on multiuser MIMO in the standardization (e.g., see Non-Patent Literature 2 and Non-Patent Literature 3).
FIG. 1 shows a downlink frame format discussed in the IEEE802.16m standard draft. FIG. 2 shows an example of MU-MIMO allocation information corresponding to n-th terminal apparatus MS#n. FIG. 3 shows a configuration of a base station apparatus (hereinafter, may be simply described as “base station”) and terminal apparatus (hereinafter, may be simply described as “terminal”) that perform MU-MIMO transmission on a downlink based on the discussion on the IEEE802.16m standard draft.
FIG. 1 shows a frame format when time division (TDD) transmission is carried out. When transmitting data specific to a terminal (or user) in a specific data region of a downlink (region represented by “DL” in FIG. 1), a base station apparatus transmits the data by including resource allocation information to the terminal in specific control information transmitted to terminals in the area. In the IEEE802.16m standard in particular, resource allocation information is included in a region allocated as A-MAP (Advanced MAP). As shown in FIG. 2, resource allocation information RA#n for MS#n includes information on the position of data transmission region (DL-burst) to a specific user, allocation size and distributed/concentrated arrangement. Furthermore, MIMO mode information included in the specific control information indicates transmission information such as a spatial multiplexing mode or time/space diversity transmission mode. In the case of a MU-MIMO mode, the MIMO mode information further includes pilot sequence information PSI#n and the total number of spatial streams Mt during MU-MIMO communication. Furthermore, MCS information included in specific control information indicates an M-ary modulation value applicable to a spatial stream to terminal MS#n and information on the coding rate. Furthermore, MCRC included in the specific control information is CRC information masked with terminal identification information (CID: connection ID) assigned when a connection is established. With this, the terminal detects specific control information addressed to the terminal along with error detection.
The base station apparatus individually notifies MU-MIMO allocation information to terminals using the above-described downlink specific control channels prior to MU-MIMO transmission. As shown in FIG. 2, the MU-MIMO allocation information includes information on the number of spatial streams (Mt), MCS#n which is information on a coding rate of an error correction code applied to a spatial stream addressed to MS#n and modulation, pilot information (PSI#n) addressed to MS#n and resource allocation information RA#n addressed to MS#n, as parameters necessary for reception processing on the terminal MS#n side. Here, n=1, . . . , Mt. That is, a case is assumed here where one spatial stream is allocated to one terminal.
Here, distributed/concentrated arrangement information, position (start, end) information, allocation size information or the like are included as resource allocation information as described above.
In the IEEE802.16m standard draft, resources are arranged based on a physical resource unit (PRU) made up of a predetermined number of OFDM symbols and subcarriers. The predetermined number of pilot signals are arranged within the PRU. FIG. 4 shows one configuration example of the PRU in 2-stream transmission. Here, the PRU is made up of six OFDM symbols in the time direction and 18 subcarriers in the frequency direction. The PRU includes 12 pilot symbols and 96 data symbols.
There are two types of resources arrangement methods; concentrated arrangement and distributed arrangement. In the concentrated arrangement, subcarriers having relatively good receiving quality are continuously allocated as resources for a terminal, based on a receiving quality situation from the terminal. This is a resource arrangement method especially suitable for when the moving speed of the terminal is slow and a time variation of receiving quality is moderate. On the other hand, in the distributed arrangement, resources distributed on subcarriers are allocated to the terminal to make it easier to obtain a frequency diversity effect. This is a resource arrangement method especially suitable for when the moving speed of the terminal is fast and a time variation of receiving quality is violent.
1) Concentrated Arrangement (Continuous RU or Localized RU)
User specific data individually transmitted to terminals (specific data or user specific data) is allocated to a physical resource PRU based on the unit of a logical resource unit (LRU: Logical RU). Here, the LRU includes data corresponding to a number of data symbols other than pilot symbols included in the PRU. The LRU configuration data is allocated to data symbol arrangement portions in predetermined order in the PRU. Furthermore, resources are allocated to user specific data based on the unit of one PRU (that is, in mini-band units) or based on the unit of n (n≧2) PRUs (that is, in subband units). FIG. 5 shows an example of resource concentrated arrangement using subbands with n=4.
2) Distributed Arrangement (Distributed RU)
The user specific data is allocated to a physical resource PRU using a logical resource unit (LRU: Logical RU) as a minimum unit. A plurality of items of LRU configuration data are arranged in a plurality of PRUs in a distributed manner according to a predetermined rule through subcarrier interleaving (or tone permutation). When a transmission diversity technique such as SFBC (Space-Frequency Block Coding) is applied, distributed arrangement is performed using two subcarriers as one unit to secure continuity between two subcarriers. That is, 2-subcarrier-based interleaving (or 2 tone based permutation) is performed. FIG. 6 shows an example of the distributed arrangement in this case.
Furthermore, a spatial stream addressed to terminal MS#n is formed by precoding modulated data signal #n addressed to terminal MS#n and pilot signal #n using common precoding weight #n. Mt spatial streams addressed to the terminal are spatially multiplexed. That is, Mt spatial streams addressed to the terminal are mapped to predetermined resources, OFDMA-modulated and transmitted. In this case, a precoded MIMO propagation channel can perform channel estimation using pilot signals precoded with the same precoding weight as that of data signals. Therefore, precoding information is unnecessary for MU-MIMO mode information. Furthermore, through frequency division multiplexing, pilot signals become orthogonal to each other between spatial streams. Therefore, MIMO propagation channels can be estimated by the terminal on the receiving side.
On the other hand, the terminal performs the following reception processing. First, the terminal receives a downlink-specific control channel and detects MU-MIMO allocation information addressed to the terminal. That is, the terminal extracts data of resources allocated for MU-MIMO transmission from the data subjected to OFDMA demodulation processing. The terminal then performs channel estimation on the MIMO propagation channel using pilot signals corresponding to the number of spatial streams (Mt).
The terminal then generates a reception weight based on the channel estimation result and pilot information (PSI) addressed to the terminal. In this case, linear reception processing such as an MMSE algorithm is performed. The terminal then separates streams addressed to the terminal from data of resources allocated to the terminal using the generated reception weight.
After separating streams addressed to the terminal, the terminal performs demodulation processing and decoding processing using the MCS information.
Furthermore, when terminal MS#n can perform maximum likelihood estimation (MLD) reception capable of obtaining high receiving quality, terminal MS#n performs MLD demodulation using modulation information of simultaneously and spatially multiplexed spatial streams addressed to other users (e.g., QPSK, 16 QAM, 64 QAM or the like). This other-user-related modulation information is included in specific control information. As disclosed in Non-Patent Literature 5, an MLD reception method generates a replica using channel estimate value H of a MIMO propagation channel and transmission signal candidate Sm, and determines a signal candidate that minimizes a Euclidean distance from received signal r as a transmission signal. Therefore, not only modulation information of spatial streams addressed to the terminal but also modulation information Mp of spatial streams including those addressed to other users are necessary to provide transmission signal candidate Sm used to generate a replica. This other user modulation information Mp is notified using two bits per one other user as shown, for example, in FIG. 7. When performing multiuser MIMO transmission, this makes MLD reception applicable to reception processing by the terminal, and can thereby improve receiving quality of the terminal.
Furthermore, according to the IEEE802.16m standard draft, a base station adopts a resources allocation method of periodically allocating the same resource to a terminal (e.g., see Non-Patent Literature 4). This allocation method is called “Persistent Allocation (PA).” The Persistent Allocation (PA) will now be described with reference to FIG. 8. FIG. 8 illustrates resources allocated to a downlink (DL) and uplink (UL) in the case where PA is applied to TDD transmission.
In a DL of FIG. 8, “PA-MAP” allocated to a k-th frame is a downlink allocation control channel for allocating downlink allocation control information of PA and a base station notifies the PA allocation target terminal of a PA initiation (resource allocation initiation) instruction using PA-MAP. Furthermore, in the DL of FIG. 8, “PA1” allocated to the k-th frame is a downlink data channel addressed to the PA allocation target terminal, and in the example shown in FIG. 8, the downlink data channel addressed to the PA allocation target terminal is periodically allocated in a period of N frames. Here, N is a frame unit repetition period and is a parameter indicated by PA-MAP. The above-described downlink allocation control channel (PA-MAP) is notified from the base station to the terminal at the events of “PA initiation (PA),” “PA reallocation” and “PA deallocation.” FIG. 8 shows an example where the base station transmits PA-MAP that instructs “PA initiation” in a k-th frame to the terminal and transmits PA-MAP that instructs “PA deallocation” in a (k+m×N)-th frame. Here, m indicates an arbitrary integer value.
PA-MAP transmitted at the event of “PA initiation” contains information on a period of the downlink resource for allocating the downlink data channel, a position and size of the downlink resource, and an uplink resource (hereinafter also referred to as “data response resource”) to feed back ACK/NACK (Acknowledgment/Negative Acknowledgment) which is a response signal for the downlink data to the base station or the like. The terminal receives the downlink data based on information on period N and the position of the downlink resource included in PA-MAP and transmits ACK/NACK in response to the received downlink data using the data response resource.
HF or HFA (HARQ Feedback Allocation) can be used as information indicating an uplink data response resource. HF indicates a resource number of the data response resource. An uplink channel through which downlink data and ACK/NACK for downlink allocation control information are transmitted is called a feedback channel (FBCH or HFRCH (HARQ Feedback Channel)).
For example, as shown in FIG. 8, when the base station transmits HF1 in a k-th frame at the event of “PA initiation,” the terminal transmits ACK/NACK in response to downlink data using a resource of HFBCH corresponding to HF1.
PA-MAP that notifies a “PA termination” instruction includes information on an instruction that PA allocation terminates (PA deallocation) and an uplink resource for feeding back ACK/NACK which is a response signal to the “PA termination” instruction to the base station (hereinafter also referred to as “control response resource”) or the like. The “PA termination” instruction (PA deallocation) is notified together with information of the allocation space allocated by the PA initiation instruction. As the information indicating a control response resource, HF can be used as in the case of the data response resource. Here, for the control response resource, a resource different from the data response resource allocated at the event of “PA initiation” is instructed. That is, suppose HF1 at the event of “PA initiation” and HF2 at the event of “PA termination” instruction have different values.
When the terminal normally receives PA-MAP transmitted from the base station and recognizes it to be a PA termination instruction (PA deallocation), the terminal transmits ACK to the base station as a normal reception response to the PA termination instruction using the control response resource instructed by HF2.
To be more specific, as shown in FIG. 8, when the base station instructs HF2 with PA-MAP that notifies a “PA termination” instruction in a (k+m×N)-th frame, the terminal transmits a response signal (in this case, only ACK) to the “PA termination” instruction using a resource of HFBCH corresponding to HF2. Since downlink data does not exist at the event of “PA termination,” the terminal does not need transmission of ACK/NACK in response to the downlink data. Thus, at the event of “PA termination,” the terminal transmits a response signal (ACK only) to the downlink allocation control information (“PA termination” instruction) using a resource different from the data response resource allocated at the event of “PA initiation” using the control response resource.
On the other hand, on the base station side, retransmission control (error handling processing, Error Handing) of the “PA termination” notification is performed as follows. That is, the base station detects response signals in HF1 at the event of “PA initiation” notification and in HF2 at the event of “PA termination” notification, and when a response signal at the event of the “PA termination” instruction cannot be detected as an ACK signal with the resource of HFBCH specified by HF2, retransmission control of the “PA termination” instruction is performed. This is because when the terminal cannot normally receive the “PA termination” instruction (that is, at the event of overlooking or reception NG), a response signal is transmitted using the data response resource instructed with HF1 at the event of “PA initiation” and a response signal to the “PA termination” instruction (in this case, ACK only) is transmitted using the resource of HFBCH corresponding to HF2 only when the “PA termination” instruction is normally received.
Next, FIG. 9 shows an example where the base station transmits PA-MAP for instructing “PA initiation” to the terminal in a k-th frame and transmits PA-MAP for instructing “PA re-allocation” to change the position of resource allocation for downlink data transmission in a (k+m×N)-th frame.
The PA-MAP transmitted at the event of “PA re-allocation” includes information on the position and size of the downlink resource of the downlink data channel for performing re-allocation, the period of the downlink resource and data response resource for feeding back ACK/NACK (Acknowledgment/Negative Acknowledgment) which is a response signal to the downlink data to the base station or the like. The terminal receives the downlink data based on information on period N and the position of the downlink resource included in PA-MAP and transmits ACK/NACK in response to the received downlink data using the data response resource. Here, for the data response resource, a resource different from the data response resource allocated at the event of “PA initiation” is instructed. That is, suppose HF1 at the event of “PA initiation” and HF2 at the event of a “PA re-allocation” instruction have different values.
To be more specific, as shown in FIG. 9, when the base station instructs HF2 in PA-MAP that notifies a “PA re-allocation” instruction in the (k+m×N)-th frame, the terminal transmits ACK/NACK in response to downlink data (PA2) of the downlink resource specified with “PA re-allocation” using the resource of HFBCH corresponding to HF2.
Furthermore, since downlink data channels are periodically allocated in a period of N frames in following frames, the terminal transmits ACK/NACK in response to the downlink data (PA2) at the position of the downlink resource instructed by “PA re-allocation” for every N frames using the resource of HFBCH corresponding to HF2.
On the other hand, the base station side performs error handling processing in response to “PA re-allocation” notification as follows. That is, the base station side detects response signals in HF1 at the event of “PA initiation” notification and HF2 at the event of “PA re-allocation” notification, and performs retransmission control of the “PA re-allocation” instruction when the response signal at the event of the “PA re-allocation” instruction cannot be detected as an ACK signal or NACK signal with the resource of HFBCH specified by HF2. This is because when the terminal cannot normally receive the “PA re-allocation” instruction (at the event of overlooking or reception NG), a response signal to data (PA1) at the downlink data position specified at the event of “PA initiation” notification is transmitted using the data response resource instructed with HF1 at the event of “PA initiation” and only when the “PA re-allocation” instruction is normally received, a data response signal for the “PA re-allocation” instruction is transmitted using the resource of HFBCH corresponding to HF2.